The invention relates to a process for production of a clean gas that is provided in particular for testing a pressurized construction component for leaks, in particular a process for production of pure nitrogen, oxygen, argon or carbon dioxide, which is free of any other gases, in particular noble gases such as helium. The invention also relates to uses of the clean gas. Further, the invention concerns a noble gas free, in particular a helium-free gas composition and its use. The invention further relates to a process for performing leak tests on a construction component pressurized with a test gas such as a process for performing leak tests with localization of leaks within an accessible testing chamber, which is flooded with noble-gas-free air. The invention particularly relates to pressurization of containers, for example a vacuum vessel of a particle accelerator, with a noble-gas-free clean gas, with a slight overpressure, in order to prevent atmospheric noble gas from flowing into the vessel.
Testing the tightness of construction components such as containers, receptacles, reactors or similar items, represents a very important task in vacuum engineering and for other engineering technologies such as clean room technology and the design of liquid containers. The flushing gas method is often used for testing the tightness in which, for example, the inner surface of a construction component has pressure applied to by a pressurized test gas and an opposing, external surface of the construction component is flushed with flushing gas in a locally limited gas space. If the construction component is leaking the test gas can pass into the flushing gas and be detected in it by using a mass spectrometer. Applications of the flushing gas method are described, for example, in DE 2 009 197 (testing fuel tanks), DE 24 137 070 C2 (testing construction components with joints) and in DE 10 2006 016 747 A1 (leak testing in vacuum engineering). Gas used as a test gas (or: leak gas, test gas) is typically helium, while the flushing gas comprises a technical gas or a gas mixture, such as technical nitrogen or air.
The sensitivity of the flushing gas method, in particular the detection limit and/or the detection speed, depends on the purity of the flushing gas. If the flushing gas as such contains traces of the test gas the detection of the test gas escaping through a leak is aggravated. Therefore, to obtain high detection sensitivity it is of interest to have a flushing gas with the least possible concentration of test gas in it, in particular nitrogen with negligible quantities of helium in it.
Various chemical reactions are known to produce chemically pure nitrogen such as conversion of ammonium nitrite which are not, however, suitable for preparation of flushing gas in the quantities needed in practice due to the costs involved and the yields achievable. Nitrogen can also be produced through fractionation of liquid air (air separation). Details of the air separation are explained below with reference to FIG. 7. The disadvantage, however, is that the nitrogen gas obtained from air separation still contains helium. Even commercially available high-purity gases, such as “Nitrogen 7.0” (manufacturer: Linde AG, Germany) is not free from helium and is therefore only of limited value for use for high sensitivity tightness testing. It has been shown in practice that the conventional technical gases used to detect the tightness of construction components are contaminated with helium so the sensitivity of conventional testing is limited.
Only when clean gases were produced for the electrical industry it was possible to reduce the concentration of volatile gases. Thus a process is described in DE 196 40 711 A1 which allows to reduce the concentration of volatile gases in nitrogen into the ppb range. DE 693 12 843 T2 describes a process for providing technical nitrogen containing impurities in the order of 100 ppb. While these processes allow for a reduction in the residual concentration of volatile gases, the remaining concentration of the volatile gases still being too high for use as a flushing gas for precision testing of tightness.
A process has been proposed by J. H. Robertson (“J. Sci. Instrum.”, Volume 40, 1963, page 506) for the production of water-free nitrogen, by evaporating liquid nitrogen locally and feeding the released nitrogen to the respective application, for example flushing in low temperature X-ray diffraction experiments. Also this process has the disadvantage that the evaporated nitrogen is indeed water-free but still contains helium impurities. The helium concentration in the nitrogen is in a very similar concentration to that found in atmospheric air. Thus the water-free nitrogen is only of limited suitability for use for high sensitivity leak testing.
The fact that the previously used, technically produced flushing gases have been contaminated and, for example, the contamination from helium in nitrogen is roughly in the same order as of that found in air (about 5 ppm) means that the sensitivity of conventional leaks tests is about 5*10−7 Pa*l/s.
There is a need for extremely pure flushing gases for practical applications in vacuum engineering or for other tasks in the leak testing area, in particular for large volume construction components, which allow determination of the tightness of a construction component with a sensitivity of up to 10−13 Pa*l/s.
For particularly large construction components such as a plasma vessel in a nuclear fusion experiment, a high-purity room or a ship's hull, there is some interest in designing the local gas space (testing chamber) in such a way that a person can enter it. This allows targeted and rapid localization of leaks in the container but is based on the prerequisite that a breathable physiological gas is used as flushing gas. The fact that only air with the naturally occurring concentration of helium is available as a flushing gas for conventional tightness testing with accessible testing chambers means that the detection limits for the tightness testing has been limited until now to relatively high values (see above).
Another known technique for tightness testing is the vacuum method in which, for example, a high vacuum is generated on the inner surface of the construction component to be tested and the outer surface is pressurized with a test gas. Passage of the test gas into the inner vacuum can, for example, be detected using a mass spectrometer. It is true that a significantly higher leak rate of 10−13 Pa·m3/s can be achieved using the vacuum method. However, the vacuum method does have the significant disadvantage that the construction component must be evacuable. Thus thin-walled construction components cannot be tested using the vacuum method. Furthermore, leaks cannot be localized or only localized within certain limits using the vacuum method.
The above-mentioned limitations for the production of flushing gas which is free of test gas do not only exist for the combination of nitrogen as flushing gas with helium as test gas but also exist, depending on the concrete application of a leak test, for other gas combinations.
The objective of the invention is to provide an improved process for the production of a clean gas which is suitable for use as a flushing gas and allows overcoming the disadvantages and limitations of the conventional techniques. The objective of the invention is also to provide a correspondingly improved device for the production of a clean gas. Further, the objective of the invention is to provide an improved clean gas which particularly allows the use of the flushing gas method with an improved detection sensitivity. The objective of the invention is also to provide an improved process for leak testing with which the disadvantages of the conventional technique are avoided and which is characterized by a significantly improved detection limit. The invention is to develop generic overpressure processes for recognizing or evaluating leaks in such a way that even the smallest concentrations of a test gas, for example helium, down to 10−13 Pa·m3/s can be determined by a modified leak detector.
These objectives are solved by the process, the device and the gas composition of the invention arise from the dependent claims.